The hybrid between Equisetum hyemale and E . laevigatum , E . × ferrissii , was mistaken for E . laevigatum by Schaffner and some subsequent authors. Although sterile, it exists outside the range of E . laevigatum , and apparently it is dispersed vegetatively (R.L. Hauke 1963). Perhaps it has persisted in some areas from a time when the parents were both there. Equisetum × ferrissii has been reported from Maine, New Hampshire, Rhode Island, South Dakota, and Vermont, but I have not seen specimens from those states.